Carbon emissions are acidifying oceans at a faster rate than at any time in the past 300 million years, raising the prospect of ecological catastrophe in decades to come, say scientists.

Carbon emissions are acidifying oceans at a faster rate than at any time in the past 300 million years raising the prospect of ecological catastrophe in decades to come, scientists have warned.

When seawater becomes too acid, corals and shrimp-like plankton at the bottom of the food chain cannot survive.

The knock-on effects can lead to widespread mass extinction of marine species - and is believed to have done in the past.

In the last 100 years atmospheric carbon dioxide has risen to about 30% above pre-industrial levels.

At the same time, the pH of the oceans has fallen by 0.1 unit to 8.1. PH is a measure of acidity – the lower the figure, the more acid a body of liquid is.

The Intergovernmental Panel on Climate Change (IPCC) predicts that ocean pH may fall another 0.3 units by the end of the century to 7.8.

New research has shown that even during periods of past mass extinctions the ocean acidity rate nowhere near matched what it is is today.

Scientist looked to the past to get a better picture of what is now happening in the oceans.

Hundreds of previous studies of climate change events in the past 300 million years were reviewed.

On only one occasion did ocean acidity increase even remotely as fast as it is today, the Palaeocene-Eocene Thermal Maximum (PETM) around 56 million years ago.

In the early 1990s scientists discovered that during this period, a mysterious doubling of carbon dioxide concentrations in just 5,000 years raised global temperatures by 6C.

There was also clear evidence of the effects of ocean acidification. A brown layer of mud was all that remained of carbonate plankton shells dissolved by the acidic waters.

As many as half of all species of benthic foraminifers, a group of single celled organisms dwelling at the bottom of the oceans, disappeared.

Although no other major extinctions are known to have occurred during the PETM, the event dramatically changed the ecological landscape.

During the PETM scientists estimate that ocean pH may have fallen by as much as 0.45 units.

Today, the oceans are acidifying at a rate at least 10 times faster than occurred then. However, it may take decades before the effects on marine life show themselves, said the scientists.

The research, published in the journal Science, also looked at two other catastrophic climate change events at the end of the Permian and Triassic periods 252 million and 201 million years ago.

Both were triggered by bouts of massive volcanism and had an enormous impact, but over a longer period of time than the PETM.

The Permian mass extinction wiped out 96% of marine life. Although scientists have not been able to reconstruct ocean pH levels at this time, they have found evidence of ocean "dead zones" and the survival of organisms able to withstand more acidic conditions.

At the end of the Triassic period, a doubling of atmospheric carbon saw the collapse of coral reefs and a second decimation of life in the oceans.

Professor Andy Ridgwell, from the University of Bristol, a member of the international research team, said: "The geological record suggests that the current acidification is potentially unparalleled in at least the last 300 million years of Earth history, and raises the possibility that we are entering an unknown territory of marine ecosystem change.

"Although similarities exist, nothing in the last 300 million years parallels the rates of future projections in terms of the disrupting of ocean carbonate chemistry – a consequence of the unprecedented rapidity of CO2 (carbon dioxide) release currently taking place."

Laboratory studies have shown that lower pH levels can harm a range of marine life, from reef and shell-building organisms to the tiny snails favoured by salmon.

In ocean pockets acidified by underwater volcanoes venting carbon dioxide, scientists have seen alarming signs of what may be to come.

One study of coral off Papua New Guinea, published in the journal Nature Climate Change last year, showed that when pH dropped to 7.8 reef diversity fell by up to 40%.

Other research has shown that laboratory-raised clownfish larvae lose their ability to detect predators or find their way home when pH falls below 7.8.

Dr Christopher Langdon, from the University of Miami, US, a co-author of the Papua New Guinea study, said: "It’s not a problem that can be quickly reversed. Once a species goes extinct it’s gone forever. We’re playing a very dangerous game."

Dr Barbel Honisch, from Columbia University in New York, US, who led the new research, said: "What we’re doing today really stands out.

"We know that life during past ocean acidification events was not wiped out - new species evolved to replace those that died off.

"But if industrial carbon emissions continue at the current pace, we may lose organisms we care about – coral reefs, oysters, salmon."

Dr Richard Feely, an oceanographer from the US National Oceanic and Atmospheric Administration (NOAA), who was not involved in the research, said: "These studies give you a sense of the timing involved in past ocean acidification events – they did not happen quickly.

"The decisions we make over the next few decades could have significant implications on a geologic timescale."